The human spine is a system which consists of a succession of vertebral bodies which, in its normal state, extends within and defines a single sagittal plane. Ideally, there should be substantially no deviation in the frontal plane (perpendicular to the sagittal plane from a straight line. Within the sagittal plane, a certain degree of lumbar lordosis and thoracic kyphosis is normal and desirable. An excess degree of lumbar curvature is known as hyperlordosis, while an abnormally flat lumbar succession of vertebral bodies is known as hypolordosis. In like manner, hyperkyphosis (most commonly seen in Scheuermann's disease) is that condition evidenced by a greater-than-normal degree of curvature in the thoracic spine which gives a hump-back type appearance.
Scoliosis may be defined as lateral deviation and rotation of a series of vertebrae from the midline anatomic position of the normal spine axis. The deformity occurs in three planes--frontal, sagittal and transverse. Scoliosis, in its more severe embodiments, is a debilitating, if not deadly disease. With the progression of the curve, structural changes occur in the vertebrae and in the formation and contour of the rib cage. This, in turn, often threatens respiratory function and capacity. The curvature of the spine itself can pose danger to the spinal cord. Still further, the interrelationships between other thoracic and abdominal organs are changes and the normal function thereof is imperilled. Fully 80% of all scoliosis cases are idiopathic, i.e. the cause is cause unknown.
There is no present "cure" for scoliosis as such, but treatments of the symptoms have been known for some time--treatments with often-times questionable effectiveness, inherent intra-surgical danger to the patient, frequent patient discomfort and/or substantial inconvenience, and substantial likelihood of post-operative complication.
Non-surgical control of scoliosis (as distinguished from correction) is available. Such non-surgical treatments include physical therapy, biofeedback, electrical stimulation and orthosis (the Sayre cast, the Hibbs and Risser casts, the Milwaukee brace, the Boston brace and the Wilmington brace, for example). Reportedly, however, these non-surgical methods can collectively boast, at most, only a 70% success rate in arresting further progression of scoliosis in cases of proven curve progression in growing (relatively immature) spines. Many of these non-surgical methods are contraindicated in cases involving curvatures greater than a specific range (usually about 40 degrees), certain patients with physical infirmities in addition to the scoliosis, patients with certain remaining growth potential, and/or with patients who cannot be reasonably expected to rigorously follow a prescribed therapeutic regimen or to emotionally tolerate the limitations and appearances of the various braces.
Surgical intervention in the correction of scoliotic curvature presently involves spinal instrumentation and spinal fusion first pioneered by Hibbs, et al..sup.1 One without the other is generally viewed as ineffective under current convention. The over-all objective of the surgical intervention is to correct the scoliosis as much as is possible, and to restore compensation of the spine with a symmetrical trunk and with head, neck and shoulders centered over the pelvis; and to stabilize the spine and prevent curve progression. The objective of the spinal instrumentation portion of surgical treatment of scoliosis is to immediately correct curvature to the degree possible and to immobilize the spine in the corrected orientation until a solid fusion has taken place. FNT .sup.1 Hibbs, R. A., Risser, J. C. and Ferguson, A. B.: "Scoliosis treated by the fusion operation." J. Bone Joint Surg., 6:3, 1924.
Problems abound with currently available spinal instrumentation. Some instrumentation occupies space needed for the bone graft placed over the posterior spine. Also, the attachment means used for spinal instrumentation inherently risks intra-operative spinal cord damage with the potential for irreversible paralysis. Still further, many spinal instrumentation systems are prone to disengagement because their attachment schemes involve screws (prone to disengage from atrophic vertebrae) or hooks which only partially encircle vertebral body projections (prone to dislodging during movement).
"Harrington Instrumentation".sup.2,3 is the posterior spinal instrumentation by which all current systems are compared. Harrington instrumentation is "segmental instrumentation" which by definition involves the interlocking of vertebral components and fixation at multiple levels for imparting corrective forces. In this system, bone-purchasing hooks are attached to posterior elements of the spine--facets, laminae, and transverse processes. Through these hooks, distraction forces are applied to the concave side of the spinal curve by the ratchet principle, and compression forces are applied on the convex side of the thoracic curve at the base of the transverse processes and adjusted by the threadnut principle. FNT .sup.2 Harrington, P. R.: Treatment of Scoliosis. Correction and internal fixation by spine instrumentation. J. Bone Joint Surg., 44-A:591, 1962. FNT .sup.3 Harrington, P. R.: Surgical instrumentation for management of scoliosis. J. Bone Joint Surg., 42-A:1448, 196.
Despite its prominence, the Harrington system has drawbacks: (1) failure of derotation of the spine as the distraction force straightens the lateral curvature; as a result the rib hump is not corrected; (2) the distraction forces of the Harrington instrumentation flatten the spine with the result that the normal lumbar lordosis is obliterated thereby producing a marked deformity; (3) Harrington instrumentation does not provide enough stability to the spine and, therefore, postoperative immobilization is required in the form of a cast or spinal orthosis. In the use of the Harrington Rod to distract spinal curves in young growing children with scoliosis when posterior spinal fusion is contraindicated: (1) the bulky nature of the Harrington instrumentation is such that it protrudes well beyond the normal dorsal contour thereby "tenting up" the overlying skin and causing breakdown problems; and (2) Harrington instrumentation without fusion is prone to loosening and fracture when serially expanded to accommodate growth in cases of implantation in children; and (3) direct surgical exposure of the osseous posterior spinal components in association with any spinal instrumentation often promotes inadvertent spinal fusion.
Another modified Harrington instrumentation system is the Moe Rod, developed by Dr. John Moe for use in growing children with scoliosis requiring surgical spinal stabilization without posterior spinal fusion. The Moe Rod is identical to the Harrington Distraction Rod, except that it is threaded at both ends to engage distraction hooks for serial expansion. The complications associated with use of the Moe Rod are similar to those described above with reference to the original Harrington Rod approach.
A form of segmental instrumentation for treatment of scoliosis was developed by Dr. Eduardo Luque of Mexico. Luque's system involved positioning two contoured smooth "L" spinal rods on either side of the posterior spinous processes and wiring them directly to the underlying laminae through wires at each vertebral level of the scoliosis.
Luque's procedure involves passing multiple wires through the spinal canal to wrap around the laminae and carries with it a substantial risk of spinal cord damage. Additional detractions from the Luque system include an attendant excessive operative time with significant blood loss and the obscuring of areas where normally one would want to effect biologic fusion. This methodology has gained meaningful acceptance only in treating paralytic scoliosis.
A relatively new system of segmental instrumentation (the Cotrel-Dubousset [C-D] Instrumentation.sup.4) addresses many of the Harrington system's shortfalls and lacks some of the drawbacks of the Luque system. Nevertheless, C-D instrumentation exhibits its own shortcomings, the principle one being that it is complex and cumbersome, with too many "moving parts". Also, implantation is extremely complex and requires the skills and experience possessed by few practitioners. It obscures the posterior spine; limiting the amount of bone graft surface for biologic fusion. FNT .sup.4 Cotrel, Y. and Dubousset, J.: New Segmental posterior instrumentation of the spine. Orthop. Trans., 9:118, 1985.
CD instrumentation is a dual rod system which the affixation of rods to and on either side of the spine through attachment to posterior elements thereof. CD instrumentation is attached by way of pedicle hooks that slide under the lamina to engage the underlying pedicle with the U-shaped edge of a hook, or through laminae hooks which are cupped around the lamina or the transverse process and are directed either proximally or distally to provide either compressive or distractive forces as are needed to act as appropriate corrective forces.
CD instrumentation includes knurled spinal rods which are contoured to the general shape of the curved spine after various compression or distraction hooks are applied at frequent intervals along the spine. Usually, the distal and proximal hooks have a hole for the rod to be threaded through them and the intermediate hooks are opened posteriorly so that the rod can be slipped into the hook itself. Cylindrical sleeves, known as blockers, will have been threaded onto the shaft and then, once the rod is placed into a hook, the blocker is pushed into the hook to trap the rod inside the hook and a compression set screw locks the rod into the blocker. The blockers are unidirectional and must be positioned so as to be directed up and down into a hook to provide proper compression to the hook so it gains purchase on the bone.
The hooks are then loosened somewhat and the rod on the convex side of the scoliotic curve is then rotated medially. This tends to correct the curve as much as possible and converts the scoliosis to a more normal kyphosis. At this point, the second rod is inserted on the concave side and a similar maneuver is performed. Once the rotation is either completed and the spine is completely straight (or partial rotation is only possible because of rigidity of the curve) then the hooks are tightened somewhat. Additional correction can be obtained by using compression or distraction pliers to move the hooks along the spinal rod at this point to provide either compression or distraction along segmental parts of the curve.
Once final positioning is obtained, as much bone as possible is maneuvered through and around the bulky instrumentation to the posterior elements of the spine which previously will have been decorticated for promoting biological fusion of the posterior spine. The final step is to apply two transverse load devices which link the two rods into a three dimensional, integrated, stable structure for maintaining proper corrective forces and desired spinal orientation and alignment.
One current form of anterior segmental instrumentation for treatment of scoliosis is the Zielke instrumentation.sup.5. This system, however, obviously involves accessing the anterior surfaces of the spinal column to attach instrumentation. This, in turn, carries a significant risk of neural and vascular injury with the accompanying risk of partial or total paralysis. FNT .sup.5 Zielke, K. and Pellin, B.: Neue Instrumente and Implantate zur Erganzung des Harrington Systems. Z Orthop. Chirl, 114: 534, 1976.
The history of scoliosis and its treatment yields as its lesson a seeming paradox: it is dangerous (and often ineffective) to treat the spine in addressing spine anomalies. This paradox has heretofore remained unsolved for two primary reasons: (1) the causation and mechanisms of scoliosis are not understood and, therefore, cannot be addressed in a preventative or even curative manner; and (2) direct management of an affected system (in this case the spine) is the traditional approach common to virtually all orthopedic procedures, a predisposition which yields a myopic view of the possible remedies as evidenced by the spinal instrumentation of the prior art.